From what’s already been said, it seems that the correct kanji-fied line is:
歩み鈍いものはない
which is actually a contuation of the line before it–together they would be:
せかいのうちに おまえほど
In the world, as much as you,
あゆみの のろい ものはない
walking-slowly-thing is not.
Or, “Nothing in the world walks as slowly as you.”
It’s actually the next line,
どうして そんなに のろいのか
that translates as “Why are you so slow?”
My full, pretty literal translation:
「もしもし かめよ かめさんよ
"Hey, tortoise, Mr. Tortoise,
せかいのうちに おまえほど
in the world, as much as you,
あゆみの のろい ものはない
nothing walks as slowly
どうして そんなに のろいのか」
Why are you so slow?"
「なんと おっしゃる うさぎさん
"If you say so, Mr. Rabbit,
そんなら おまえと かけくらべ
then I will race you
むこうの 小山(こやま)の ふもとまで
to the foot of the mountain over there
どちらが さきに かけつくか」
which one will arrive first?"
「どんなに かめが いそいでも
"No matter how much the tortoise hurries
どうせ ばんまで かかるだろう
In any case, until evening it will definitely take.
ここらで ちょっと ひとねむり
I’ll nap here for a while.
グーグーグーグー グーグー
Snore, snore, snore."
「これは ねすぎた しくじった」
“What is this, slept too much, screwed up,
ピョンピョンピョンピョン
ピョンピョンピョン
Hop, hop, hop.”
「あんまり おそい うさぎさん
"Too slow, Mr. Rabbit,
さっきの じまんは どうしたの」
The boasting from a little wile ago, what’s wrong/what about it?